Three-phase motors are used in various applications. The motors are typically energized with a power source that provides electrical energy in a variety of forms. In one form, the input signal is a sine wave with each of the three motor leads being driven with a voltage or current sine wave that has a 120 degree phase difference between each lead. In the other form, “trapezoidal commutation” is used to provide a controlled current that drives only two of the three motor leads at any point in time. Sine waves are typically used to drive larger, higher performance alternating current (AC) motors, while lower cost trapezoidal commutation is typically used for small AC motors. In some applications, an AC energy source is rectified to generate a direct current (DC) voltage that is applied to a DC motor through an inverter having half-bridges. A motor controller, such as a digital signal processor (DSP), selectively activates the half-bridges of the inverter to apply the DC voltage to the windings of the DC motor. Typically, the selective activation of the half-bridges is performed by the application of pulse-width modulated (PWM) signals to driver circuits that interface the DSP to the half-bridges of the inverter.
As a motor is operated, the motor generates heat. In particular, varying the load on a motor or the speed at which a motor is rotated causes the motor to generate heat. As a motor gets warmer, the risk of motor damage increases if operation of the motor is sustained at temperatures that may degrade motor components. Consequently, the motor controller needs to reduce or terminate supply of current to the motor in response to the temperature of the motor exceeding some temperature threshold. In previous motor controllers, a temperature sensor may be placed in a lead to a motor. The sensor generates a signal that corresponds to a temperature in the motor. The motor controller may then monitor the signal from the sensor and compare the signal to a temperature threshold. As the signal approaches or exceeds the threshold, the motor controller may alter the manner in which the controller provides current to the motor.
While the monitoring of temperature in a motor lead with a temperature sensor is a viable method of motor current regulation, that method requires the expense of a temperature sensor in the motor lead. In machines that are mass-produced, such as washing machines or dryers, the expense of a temperature sensor in each machine may add significant costs to the manufacture of the machines at the necessary production rates. Thus, measurement of temperatures within an electrical motor without the use of temperature sensors would be beneficial.